Wind turbines are required to be equipped with lightning protection systems in order to ensure that large currents in lightning bolts or lightning strokes may be conducted to the ground without damaging the components of the turbine.
Wind turbine blades may be the part of a wind turbine that are most exposed to lightning strokes and therefore it may be a task of the lightning protection system of a wind turbine to provide a save conduction path from the blades to the ground. This may be challenging because some of the components of the wind turbine may be able to rotate relative to each other. Specifically the wind turbine blades may be able to rotate around their longitudinal axis, i.e. the pitch, and at the same time the main shaft of the wind turbine, which carries the blades, may be able to rotate relative to a nacelle of the wind air turbine. Furthermore, the nacelle may be able to rotate around a vertical axis of the wind turbine to ensure that the wind turbine is always facing the wind, i.e. the nacelle may be able to rotate around its yaw axis.
Thus, the current from a lightning stroke striking a wind turbine blade passes through free rotational bearings on its way to the ground.
Pitch and yaw movements are relatively slow and limited in its extent. Therefore it may comparatively easy to provide conduction paths which bypass the pitch and the yaw bearings, for instance by using metallic brushes or rollers.
However, the main shaft of the wind turbine rotates continuously and at a higher speed than the pitch and the yaw bearings. The main shaft may be supported by the base of the nacelle through one or more massive bearings. These massive bearings are also made of metal and therefore provide a natural conduction path for the currents in the lightning strokes. However, the effective contact areas between rolling elements of the bearings and inner and outer races may be small and thus large current densities in the contact area of the inner and outer races may occur.
A high current density may generate a high temperature in the contact areas that may lead to a damage of the bearings.
EP 1 788 241 A2 discloses a method for directing a current generated by a lightning stroke. The method includes directing the current from a main shaft of the wind turbine to a break disk attached to the shaft and directing the current from the break disk to one of a spark gap and a roller mechanism coupled to a down-conductor at a ground voltage.
EP 1 568 883 A2 describes a wind energy generator comprising a propeller driven generator in a housing that is mounted on a pillar. The wind energy generator is protected against lightning strokes by a conductor system that is coupled to the ground. The wind energy generator therefore uses a slip ring and a brush to provide a connection between rotary and stationary parts.